The present invention relates to an X-ray computed tomography apparatus for reconstructing the image, i.e., a tomogram, of a slice of an inner portion of a subject on the basis of projection data on the subject in many directions.
Many recent X-ray computed tomography apparatuses have an electrocardiographic synchronization function. As shown in FIG. 1, with an electrocardiogram, characteristic waves such as P, Q, R, S, and T waves can be captured. One cardiac cycle is divided into a systole A, a diastole B, and equivalent diastoles C, D, and E. The volume of the heart varies greatly in the systole A and the diastole B, but varies slightly in the equivalent diastoles C, D, and E.
According to the electrocardiographic synchronization function, for example, projection data is acquired a predetermined period of time after the R wave. This data acquisition operation is repeated to obtain, for example, 360.degree. projection data in different projecting directions during a period during which the volume of the heart remains almost the same. A tomogram having no artifact caused by variations in the volume of the heart can be reconstructed on the basis of this obtained projection data.
This electrocardiographic synchronization function, however, demands a very long scanning time. Assume that a tomogram in the systole A is obtained by an apparatus with 750 msec/rotation. Since the systole A is generally 200 msec, a period of time four times a cardiac cycle is required to obtain 360.degree. projection data in the systole A. Since the cardiac cycle is generally one sec, the scanning time is as long as four sec.
For this reason, the subject must be kept exposed to X-rays for a period as long as four sec. In addition, if the X-ray intensity is decreased to reduce the dose of X-rays on the subject, a deterioration in image quality occurs.